The basic answer to my earlier question about why tapply won't take a
matrix as its first argument is that that's the way it's written.
However, several people did suggest a nice workaround: use a tapply on a
vector of indices, and rewrite the function to take indices.
Here is the code from Bill Dunlap's reply, which is very similar to what
several others suggested.
tapply(1:nrow(mat), x$stimulus,
FUN=function(i, mat)cor(mat[i,1],mat[i,2]), mat=mat)
Thanks to those who answered, including Nick Ellis, Bill Venables, John
Wallace, Bert Gunter, and Bill Dunlap of Mathsoft. (I may have
inadvertently deleted a reply from one other person; sorry.)
Matt Wiener
NIMH
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